Interesting one...
I think that this is a fair amount of work and would involve getting another php file to provide the product ids within the database when you are at the product details page.
How would you propose the system deals with this? ie what defines the next product?
- Is it the next product listed in the database by product id
- The next product as defined by the position marker in that category (what happens at the end of a category?)
- The next product alphabetically.
If you can answer those I am sure that the code can be worked..
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ex x-cart guru 
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